Three lenses are of powers 2.5 D, 2.0 D and -0.5 D respectively. What will be the power of combination of these lenses?

4.0 D

Many optical instruments consist of a number of lenses. They are combined to increase the magnification and sharpness of the image.

The net power (P) of the lenses placed in contact is given by the algebraic sum of the individual powers P₁, P₂, P₃, … as

P = P₁ + P₂ + P₃ + …

Here, P₁ = 2.5 D, P₂ = 2.0 D, P₃ = -0.5 D

P_net = 2.5 + 2.0 + (-0.5) = 4.0 D